Fix #3641: Can't add tags via Add Commentary dialog.

app/models/artist_commentary.rb

@@ -108,7 +108,7 @@ class ArtistCommentary < ApplicationRecord
       post.add_tag("check_commentary")
     end
 
-    post.save if post.saved_change_to_tag_string?
+    post.save if post.tag_string_changed?
   end
 
   module VersionMethods

test/unit/artist_commentary_test.rb

@@ -60,6 +60,11 @@ class ArtistCommentaryTest < ActiveSupport::TestCase
         @artcomm.reload
       end
 
+      should "add tags if requested" do
+        @artcomm.update(translated_title: "bar", add_commentary_tag: "1")
+        assert_equal(true, @post.reload.has_tag?("commentary"))
+      end
+
       should "not create new version if nothing changed" do
         @artcomm.save
         assert_equal(1, @post.artist_commentary.versions.size)